1. Field of the Invention
The present invention relates to an optical fiber cable which can be used for fiber to the home (FTTH) services and the like, in which optical fibers are extended to individual homes, and also relates to a resin composition that is used therefor.
Priority is claimed on Japanese Patent Application No. 2008-294665, filed Nov. 18, 2008, the content of which is incorporated herein by reference.
2. Description of the Related Art
FIG. 1 and FIG. 2 are diagrams showing examples of an optical fiber cable used in the FTTH services and the like.
FIG. 1 shows an optical fiber cable used for introducing an optical line to a subscriber's home. This optical fiber cable 10 has a self supporting structure in which a support wire portion 1 and a cable body portion 2 are connected via a neck portion 3.
An optical fiber cable 20 shown in FIG. 2 is used, for example, for the wiring in subscribers' homes and in premises such as buildings and apartments, and it does not include the support wire portion 1 and is formed solely of the cable body portion 2.
In the optical fiber cables 10 and 20, a halogen-free, flame retardant polyolefin or the like is used for a covering resin 6.
Various proposals have been made regarding the materials constituting an outer covering of an optical fiber cable (for example, refer to Patent Documents 1 to 5).
In Patent Document 1 (Japanese Patent Publication No. 3915016) and Patent Document 2 (Japanese Patent Publication No. 3970499), use of a material containing a fatty acid amide has been disclosed.
In Patent Document 3 (Japanese Unexamined Patent Application, First Publication No. 2002-313153), a material in which a silicone dispersed polyethylene, a fluororesin dispersed polyethylene and a silicone grafted polyethylene are added has been disclosed.
In Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. 2008-94922) and Patent Document 5 (Japanese Unexamined Patent Application, First Publication No. 2008-63458), use of a polyolefin-based resin and a phosphate salt has been disclosed.
In many cases, the optical fiber cable used in subscribers' homes and in premises such as buildings and apartments is extended through a tubing for electric wires, such as a flexible tube made of a synthetic resin.
However, the existing tubing for electric wires already accommodates a plurality of metal wires, such as a telephone line, that extend therein, and thus there is only a small empty space left for the optical fiber cable which is to be additionally laid.
For this reason, there have been demands for reduction of the cable diameter and improvements in the line receiving properties.
However, when the diameter of a cable is reduced, the mass of resin per unit area reduces, and thus a highly flame retardant resin has to be used as a cable covering material. In addition, in order to improve the line receiving properties, the lowering of the friction properties of outer coverings as well as the abrasion resistance thereof against other cables (such as the existing telephone lines) or cable receiving rods become important factors.
As disclosed in Patent Documents 1 and 2, for lowering the friction properties of outer covering materials, it is effective to add a fatty acid amide to the outer covering materials. Fatty acid amides such as stearamide, oleamide and erucamide have low molecular weights, and thus easily bleed out on the surface of outer coverings, thereby making the surface of outer coverings slimy and reducing the coefficient of friction thereof.
However, the low molecular weight fraction that bled out on the surface is readily removed by the rubbing of outer coverings or the like, thereby increasing the coefficient of friction at times. Moreover, the fatty acid amides enter inside the covering UV-curable resin of an optical fiber and adversely affect the glass optical fiber, which may deteriorate the transmission characteristics.
From the above viewpoints, as a method for lowering the friction properties, it is desirable to adopt a non-bleeding system, as disclosed in Patent Document 3, in which a silicone dispersed polyethylene is blended.
Further, in addition to the use of a lubricant component as described above, it is also effective to use a resin that originally has a low coefficient of friction, such as a high density polyethylene, for a base resin. However, a highly crystalline resin such as a high density polyethylene has low filler acceptability, and thus a flame retardant cannot be added in large amounts, making it difficult to achieve a target level of flame resistance.
On the other hand, the optical fiber cable used in subscribers' homes and in premises such as buildings and apartments is not only extended through a tubing for electric wires, but also may be disposed in a cable rack or the like at times. Accordingly, from the viewpoint of preventing the spread of fire through the optical fiber cable, a flame resistance which satisfies the criteria set in an inclined combustion test specified in JIS C3005 or the like is required.
In addition, there is a demand for the use of a halogen-free optical fiber cable in consideration of the impacts on the environment and the safety of gas generated at the time of combustion. Conventionally, in order to satisfy the criteria set in the inclined combustion test using a halogen-free resin, it was necessary to add a large amount of magnesium hydroxide, aluminum hydroxide or the like as a flame retardant.
It has been technically difficult to add these flame retardants to a highly crystalline resin, such as a low density polyethylene (LDPE), a linear low density polyethylene (LLDPE) and a high density polyethylene (HDPE), in large amounts, and in order to develop the flame resistance, magnesium hydroxide, aluminum hydroxide or the like has to be added while using an amorphous resin, such as an ethylene-vinyl acetate copolymer (EVA) and an ethylene-ethyl acrylate copolymer (EEA), as a base polymer.
However, the resin prepared by adding a large amount of magnesium hydroxide, aluminum hydroxide or the like to an ethylene-vinyl acetate copolymer (EVA), an ethylene-ethyl acrylate copolymer (EEA) or the like has a high coefficient of friction, as compared to a low friction polyethylene, a linear, low friction polyethylene and a high density polyethylene, and also, a satisfactory level of low frictional properties cannot be attained even when a silicone dispersed polyethylene or the like is added thereto.
Moreover, as disclosed in Patent Document 4, when the amount of filler added increases, the abrasion resistance deteriorates, thereby making the resin to easily abraded out, or the cold resistance (embrittlement characteristics) of resin is impaired. In addition, it should be noted that in order to enhance the abrasion resistance, it is effective to use a high density polyethylene, which primarily has a favorable level of abrasion resistance, as a base resin. However, in this case, the filler acceptability reduces, and thus a required level of flame resistance cannot be ensured at times.